This invention relates to an improvement of an easy-open can cap with a ring pull tab and with a deformed ellipse-shaped tear-opening portion scored on the cap surface, for a variety of drinking cans made of tinned or tin-free steel plates.
In such a conventional easy-open can cap (30) with a ring pull tab as shown in FIG. 1, after charging the can with a drinking liquid the air in the head space is removed and the can cap (30) is seamed and sealed to a can body (32) under reduced pressure, the cap surface (34) as a whole bends downwardly while describing a gentle circular arc as is shown in FIG. 2.
However, when subjected to heating in the sterilizing step after sealing, the cap surface (34) as a whole bends upwardly while describing a gentle configuration because of an increase in the internal pressure as is illustrated in FIG. 3. As a result, a ring pull tab (42), which is riveted with a rivet (36) to a tear-opening portion (38) in an approximately central portion of the can cap (30) and which extends near a seam band (40), rises highest at the position of the rivet (36) as is illustrated in FIG. 3 due to the upward bending of the cap surface (34). Also a finger hook (44) of the ring pull tab (42) rises higher than the seam band (40) until the can body (32) is again reduced in pressure and this causes difficulties in the conveyance of cans, for example, the finger hook (44) will be hooked to the conveyor.
When the can body (32) is cooled and again reduced in pressure after the sterilizing process, the can surface (34) bends downwardly as is illustrated in FIG. 2, so that the centrally located rivet (36) assumes the lowest position, and the riveted ring pull tab (42) also goes down so the finger hook (44) approaches the cap surface (34), resulting in that, when opening the can, not only it is difficult to insert a finger in the finger hook (44) but also the nail tip may be damaged.
In the conventional can cap (30) of this sort as shown in FIG. 1, the opening operation for the tear-opening portion (38) which is scored in the form of a deformed ellipse on the cap surface (34) with a score (46), is begun with the initial opening (score break) operation (see FIG. 5) in which the finger hook (44) of the ring pull tab (42) is pulled up with a finger (see FIG. 4) and is then pivotally moved with both side fulcrum ends (48a) and (48b) of the ring pull tab (42) as fulcrums to thereby break a small-diameter semicircle portion (46a) of the score (46), then the operation shifts to the second opening (score tear) operation in which the ring pull tab (42) is pulled up with a nose portion (50) as a fulcrum (see FIG. 6) and the tear-opening portion (38) is torn and opened along with both sides straight-line portions (46b) and (46c) of the score (46).
In the can cap (30) of this sort, as is illustrated in FIG. 7, since the score (46) is stamped to form the tear-opening portion (38), the latter portion becomes outwardly convexed due to the plastic flow phenomenon of the surplus metal at the time of the stamping operation and thus a ridge portion (52) is formed in the axial larger diameter direction of the tear-opening portion (38).
In the tear-opening portion (38) having such ridge portion (52) in the axial larger diameter direction thereof, when one tries to effect the initial opening operation in the small-diameter semicircle portion (46a) with the fulcrum ends (48a) (48b) as fulcrums, it is difficult to bend the tear-opening portion (38) because of the existence of rigidity in the opening direction created by the ridge portion (52) of the tear-opening portion (38). For this reason, as is shown in FIG. 5, an initial opening area (W) becomes large and a force as large as 2.0 to 3.0 kg. is required to cause the initial breakage, which not only produces pain in the finger tip but also sometimes makes it impossible for women and children to open the can.
Besides, due to the relation between the depth of the score (46) and crack in tinned or tin-free steel plates, the ordinary stamping operation has a restriction such that the score residual can be only about 60-70.mu. at most.
As a result, when opening the tear-opening portion (38) of the drinking can easy-open cap (30) with the ordinary standard ring pull tab (42), the opening operation requiring the maximum opening force concentrates on the second opening operation, particularly at the beginning thereof, as is shown in FIG. 8; in other words, as compared with the breaking force as large as 2.0 to 3.0 kg. required for the initial opening operation, the tear starting force in the second opening operation reaches an even larger value of at least 3.0 to 5.0 kg.
In the easy-open can cap of this invention with a ring pull tab riveted in a small-diameter semicircle portion of a tear-opening portion which is defined on the cap surface by a deformed ellipse-shaped score, a score is formed perpendicular to the axial larger-diameter direction of the said tear-opening portion and in the vicinity of a circular arc-shaped nose portion of the ring pull tab, and further the score residual in a predetermined transitional area from where the second opening operation is started by pulling up the ring pull tab after the small-diameter semicircle portion was broken by the initial opening operation until where the second opening operation becomes stable, is made smaller than the score residual at least in the said small-diameter semicircle portion.
It is an object of this invention to provide an easy-open can cap with a ring pull tab whereby the opening operation for a tear-opening portion defined on the cap surface by a deformed ellipse-shaped score can be started smoothly at a reduced initial opening force without causing breakage or coming-off of the tab.
It is another object of this invention to provide an easy-open can cap with a ring pull tab whereby the transitional tearing operation can be done smoothly with reduced second opening force halfway in the opening operation for the tear-opening portion defined on the cap surface by a deformed ellipse-shaped score.
It is a further object of this invention to provide an easy-open can cap with a ring pull tab whereby a finger can be inserted smoothly in the finger hook of the ring pull tab to pull up the latter no matter at what pressure the can interior may be.
It is another object of this invention to provide an easy-open can cap with a ring pull tab in which even when the interior of the can is at a positive pressure the finger hook of the ring pull tab does not project above the seam band between the can cap and the can body so the finger hook can be prevented from being hooked to the conveyor etc. during conveyence of the can.
Other and further objects of this invention will becomes apparent from the description of the specification and the drawings.